This invention relates to methods for transferring solid materials between regions of substantially different pressures, and more particularly it relates to the use of movable pressure seals, formed from a mixture of solid particles and liquid, that can be transferred along with the solid material from one region to the other.
The methods of the present invention are typically useful in coal gasification plants, for the purpose of feeding particulate coal from an external region under atmospheric pressure into an interior region of a gasification unit where pressures on the order of 1 to several thousand pounds per square inch gauge or more prevail. Likewise the invention makes possible improved techniques for removing hot char from a high-pressure interior region to a disposal system operating at ambient atmospheric pressure.
A number of different techniques intended to accomplish purposes somewhat similar to the foregoing results have been employed or proposed in the past. According to U.S. Pat. No. 1,974,789, materials in solid particulate form, which may be mixed with liquids, are carried in a moving chamber formed between two pistons that reciprocate together within a cylinder extending between the high and low pressure regions. While the two regions are nominally sealed off from each other by the piston rings, the moving chamber itself tends to exchange the atmospheric constituents of the two regions, and it is not likely that the piston ring seal can be effective for any substantial length of time when the cylinder and ring surfaces are subjected to the abrasive action of a solid material such as coal.
Another old proposal is contained in British Patent Specification No. 262,901, which discloses the concept of a continuous, elongated and intermittently moving stopper formed from the particulate solid material per se which is being transferred between the two pressure regions. A reciprocating plunger periodically compresses and moves forward additional quantities of the solid material which is thereby extruded under high pressure through a tubular passage extending between the two regions. This arrangement depends on the tightly packed condition of the material within the tube to prevent blow-out of the stopper when the plunger moves backward to admit another charge of material, and hence requires extremely high pressures and high power consumption by the extrusion apparatus. The patent discloses that the formation of a gas-tight stopper can be assisted by adding to the solids a suitable substance which will cause the solids to cake together to form a tight core, or by introducing into the tube at intervals tight layers of materials such as rubber, paper, cardboard or metal whereby the body of the stopper can be made gas tight. Unfortunately it appears that these extrinsic layer materials, even if physically or chemically compatible with the process, either cannot be made to form an effective seal at the inner wall of the tube, or are destroyed by the high pressure and abrasive action of particles such as coal.
In the operation of modern coal gasification units, two main techniques are used for coal feeding. One technique utilizes bulk transfer of dry coal through a lock hopper system, as described by E. J. Ferretti, "Feeding Coal to Pressurized Systems", Chemical Engineering, Dec. 9, 1974. The other technique involves continuous feeding of fine wet coal into the system by means of a slurry pump. These two techniques are subject to substantial energy losses.
The present invention provides methods for feeding solid particles from a first region at a first pressure, for example atmospheric pressure, to a second region, which may be the interior of a closed reaction vessel, where a second pressure, perhaps on the order of one thousand psig or greater, prevails. The particles are conveyed into an elongate tube which is connected to communicate between the first region and the second region. In order to minimize any leakage or interchange of their fluid contents between the two regions, for example, the escape of gas from a highly pressurized region, this invention provides a movable sealing zone in the tube. This zone comprises a mixture of solid particles and a liquid contacting the inner wall of the tube. The particles are then forced from the tube into the second region, carrying a sealing zone along with them.
The invention will be illustrated and described in the context of a coal gasification plant, as noted above, for feeding coal to the gasification unit and for removing char therefrom. However, it is apparent that the methods and apparatus have utility in any other arrangement where it is necessary to transfer particulate solids from a region of low pressure to one of high pressure or vice versa.
The use of the solid particles and liquid sealing zone enables a leak-tight seal to be made, between a column of particles being fed and the inner wall of the tube, and such a tight seal can be effected with reduced pressure in comparison with prior arrangements, thus reducing the power consumption of the mechanical feed mechanism. The liquid sealing zone can economically use a hydrocarbonaceous liquid such as lubricating oil or crude oil, which reduces friction and likewise power consumption while at the same time reducing wear on the tube and the ram or plunger which is preferably employed therein to move the column of particles. This arrangement has an obvious energy-conservation advantage over the water-slurry feeding method in that the substantially unrecoverable energy required to vaporize the water does not have to be consumed, whereas the energy contained in the oil is fully recovered in the gasifier product.
Finally, the use of the oil, which may be a petroleum product, can be reduced to a very low level by forming the zones along only very short segments of the column of particles being conveyed, such segments constituting only a very minor portion of the length of the column. Also, the solid-liquid mixture in the zones may be formed only around the periphery of the column, so that the volume of solid-liquid mixture used is only a very small fraction (typically about 0.2%) of the total volume of material being fed into the gasifier. Since this small fraction itself may contain only around one part in 10 or less of the oil, the economy of oil use is evident.
According to some of the more detailed aspects of the invention, seal plugs or rings, compression-molded from the mixture of solid particles and liquid, can be prepared outside the injection tube and transferred to the tube before the coal or other solid particles are to be forced from the tube into the second region. The plugs or rings thereby form sealing zones of relatively small extent along the length of the tube which is otherwise filled with the bulk of the dry working particles being fed through the tube.
A substantially fluid tight seal, typically comprising a gate or an auger, may be provided between the second region and at least a substantial portion of the tube communicating with the first region while particles are being conveyed from the first region into the tube. One of the above-described seal rings or plugs normally occupies the portion of the tube at the end communicating with the second region while the particles are being conveyed into the tube from the first region. The gate or auger assists in retaining the seal ring or plug in place during this operation, preventing disruption of the movable sealing zone and consequent leakage of gas from the high pressure region to the low pressure region. The gate or auger also allows the formation of a leak-tight sealing zone, or gas-tight stopper, in the injection tube, utilizing a relatively very low pressure operation of the ram or plunger by comparison with the pressures which would be required using the methods and apparatus of the prior art.
Typically the gate or the auger cooperates with the ram or plunger to effect precompression of the bulk of the particulate material within the tube before the material is forced from the tube into the second region. This also results in the application of a substantially high pressure to the solid-liquid seal ring or plug, expanding it laterally and squeezing its periphery outwardly into tight sealing engagement with the bore of the injection tube, before any sealing action is required of the seal ring or plug. The ram may be vented to cooperate with the seal ring so as to allow gas pressure, built up within the bulk material as a result of the precompression, to escape through the vent and the opening in the seal ring, rather than exerting pressure against the seal.
Further according to the invention we have discovered certain satisfactorily operational and preferred ranges of values for certain parameters of the seal-forming process, particularly as adapted for the injection of coal into coal gasification units. These parameters include sealing zone particle sizes, tube wall contact length, solid particles to liquid ratios in the sealing zone, operating pressures within the reactor which permit most effective sealing, seal liquid viscosity, and bulk coal particle sizes, as discussed herein.
Some objects of the invention are to provide improved methods and apparatus for feeding solid particles from one region to another region at a substantially different pressure without loss of fluid from the high pressure region or substantial commingling of the fluids in the two regions, to provide such a method and apparatus which can substantially reduce the consumption of energy required by both the feeding operation and the ordinary operation of the process, to provide moving seals comprising solid particles and a liquid which can be injected from the one region to the other along with the solid particles without substantially interfering with the operation of the process utilizing the particles, to provide a way of sealing an injection tube connected to communicate between the regions with maximum economy of the liquid used to form the seals and to lubricate the injection tube, to provide a way of forming a leak-tight seal between a moving column of dry particles and the injection tube which requires relatively low pressure from a ram or the like which is utilized to move the column, to provide improved methods and apparatus for feeding essentially dry coal into a gasification unit and for removing hot char therefrom, to provide improved gating arrangements for substantially leak-proof transport of solid particles between regions at different pressures and to provide an arrangement whereby a column of relatively loose particles can be precompacted before injection while minimizing detrimental effects on the seal due to gas pressure build up as a result of compaction of the particles.